Method of producing magnetically anisotropic single-crystal magnets



1967 AKIRA HIGUCHI ETAL 3,350,240

, METHOD OF PRODUCING MAGNETICALLY ANISOTROPIC SINGLE-CRYSTAL MAGNETS Filed May 28, 1964 Fig, 2

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1200 /000 800 600 400 20a INVENTORS W WW a/ama Qww ATTORNEYS United States Patent 3,350,240 METHOD OF PRODUCING MAGNETICALLY AN- ISOTROPIC SINGLE-CRYSTAL MAGNETS Akira Higuchi, Touonaka, and Toshinobu Miyamoto, Nishiuomiya, Japan, assignors to Sumitomo Special Metals Company Limited, Osaka, Japan, a corporation of Japan Filed May 28, 1964, Ser. No. 370,874 Claims priority, application Japan, July 5, 1963, 38/35,710 2 Claims. (Cl. 148-1.6)

ABSTRACT OF THE DISCLOSURE This invention relates to a method of producing magnetically anisotropic single-crystal ferroa-lloy magnets containing A1, Ni, Co, Cu and Ti as principal elements.

The requirement for the increase of the maximum energy product per unit volume of permanent magnet has become larger with the increase of demand owing to the progress of the electronic industries. It is well known that by using a hot mold or a chill plate in casting, a

crystal texture anda high magnetic anisotropy will be given to a ferroalloy magnet known as Alnico V containing 8% Al, 14% Ni, 24% Co and 3% Cu as principal elements. It is also reported that by the adoption of a recrystallization to Alnico V alloy, -a single-crystal magnet can be obtained and higher permanent magnet properties can be illustrated.

It is further reported that a ferroalloy magnet containing 610% Al, 12-20% Ni, 20-42% Co, (ll-7% Cu and less than 6% Ti and adding at one of less than 1% Zr, less than 2% Si and less than 3% Nb thereto as required, the balance being substantially iron, can obtain high coercivity by solution treatment followed annealing in a magnetic field and then aging. -It is expected theoretically and experimentally that a ferroalloy magnet known as Alnico VIII containing 7% Al, 14% Ni, 34% Co, 4% Cu and Ti as principal elements, may be made as one of the highest performance magnets among the Alnico series permanent magnet alloys. Due to its high coercivity, the maximum energy product of single-crystal Alnico VIII will be more remarkably increased than in the case of Alnico V. However, said Alnico VIII alloy contains in such larger amount of Ti as a principal element that it is very diflicult to make it a magnetically anisotropic single-crystal magnet, due to the following reasons:

(1) Its crystal grain size is so small that the average diameter of the grain as ordinally cast is only about 0.3- 0.5 mm.

(2) When a hot mold or chill plate is used, slight columnar crystals may be formed. But, in such case, too, the grain is small and short that its permanent magnet properties are not so improved.

(3) When a recrystallization for making a single-crystal magnet from poly-crystal one is applied, where the first heat treatment at a temperature of 920-1175 C. for the precipitation 'y-phase and the following heat treatment at a temperature of 1220-1320 C. for the solution treatment and the grain growth are preformed, the grain is Patented Oct. 31, 1967 Ru, Pd, etc. for accelerating 'y-phase precipitation. Be-

sides, the addition of such elements is nearly impossible from viewpoint of the production technique.

As described above, it is impossible to make industrially this kind of permanent magnet alloy single crystal by the well-known methods. As a result of further investigations for obtaining single-crystal magnet in this kind of alloy, the following new facts have been discovered.

(1) By a high frequency zone-melting method to which the principle of the hot mold-method or Bridgemans method is applied, very fine but columnar crystals can be made in this kind of permanent magnet alloy.

(2) A crystal texture is given in advance to a polycrystal magnet by the abovementioned method and the magnet is heated to 1250-1280 C. in a heating rate of 5-20 C./min. and then slowly cooled. By applying this cycle of heat treatment at least twice, the magnet can be positively made single crystal in a wide composition range.

(3) By carrying out the heat treatmentmentioned in (2) above, in the atmosphere of cracked ammonia or carbon monoxide, coarsening of crystals can be remarkably accelerated.

(4) In the above treatment, it is unnecessary to add said specific elements for promoting the 'y-precipitation.

(5) After the treatment in (1) and (2) above, by quenching the magnet from 1250-1280 C., subjecting it to standard magnetic field annealing and then aging, the maximum energy product of the magnet can reach to 11 m.g.o. (million gauss oersted).

It seems to be caused from the fact that on repeating of the heat treatment cycle, lattice relationship between the 'y-precipitate and a-matrix is always kept and y-precipitates in grain boundary play a role to reduce boundary energy and bring easily the boundary migration required for recrystallization.

Thus, the present invention provides an industrial means of making magnetically anisotropic single-crystal magnets. A ferroalloy containing Al, Ni, Co, Cu and Ti as principal elements is melted in a high frequency furnace and then cast in a mold, where the crystal texture is regulated by using a hot mold method or a high frequency zone-melting method to make a orientation. This poly-crystal texture or columnar magnet is heated to 1250-l280 C. in a heating rate of 5-20 C./min. and then slowly cooled to about 900 C. This heat treatment cycle is repeated at least twice. The magnet is then quenched from 1250-1280" C. to obtain a homogenized single-crystal magnet. Then, subjecting the magnet heated about 800 C. near to the Curie point of the magnet to the annealing in a magnetic field parallelly to the crystal orientation and then aging at about 600 C., gives permanent magnet properties excellent in the initial crystal orientation.

In the accompanying drawings;

FIGURE 1 is a diagram of a heat treatment cycle explaining the conversion of a poly-crystal magnet to a single-crystal one according to the present invention.

FIGURE 2 is a B-H curve of the single-crystal magnet based on the present invention as compared with that of a conventional poly-crystal one.

The conversion from the poly-crystal magnet to singlecrystal one caused by the heat treatment according to the present invention is as shown in FIGURE 1 and the heat treatment cycle is very effective to this purpose. Further, as the initial crystal orientation does not collapse, as shown in FIGURE 2, remarkable increases of permanent magnet properties represented by A to B in Alnico V alloy and C to D in Alnico VIII alloy can be brought by the present invention. An example thereof is now mentioned in Table I.

TABLE I Alloys Alnico V Series Alnieo VIII Series Compositions 8% Al; 14% Ni; 24% 7% A1; 14% Ni; 36%

o; 3% Cu; 0.5% Go; 3% Cu; 5.5% Si; Bal. Fe. 'l1 i; 0.5% Nb; Bal.

e. (1) Random crystal Br=12.5-13.2 kg, Br=8.59.5 kg., Hc= 15 orientation heat Hc=600-650 oe., 1,350-1,550 oe., treated in magnetic (BH)m=4.5-5.5 (BH)m =4.55.5 field and aged at m.g.o. m.g.o. 600 C. (2) Random crystal Br=13.514.5 kg. Br=8.55.5 kg., Hc=

orientation recrys- Hc=600730 oe., 1,3001,500 oe., tallized and coars- (BH)m=5.58.0 (BH) =4.55.5 ened, heat-treated m.g.o. m.g.o. in a magnetic field and aged at 600 C. (3) (100) crystal orl- Br=13.014.0 kg., Br=10.01l.0 kg.,

entation, heat- Hc=650730 oe., Hc=1,3501,550 oe., treated in a mag- (BH)m=6.5-8.0 (BH)m=6.5-8.0 netic field and aged m.g.0. m.g.o. at 600 C. (4) (100) crystal ori- Br=13.514.5 kg, Br=l1.0-12.0 kg.,

entation, single Hc=730-780 oe., He=1,4001,600 0e., crystallized and (BH)m=10.010.5 (BH)m=l0.511.5 coarsened by 3 m.g.o. m.g.o. cycles, heat-treated in a magnetic field and aged at 600 C. (according to the 0 present invention).

That 1s to say, the permanent magnet properties obtained by repeating heat treatment cycle for recrystallization at least twice after carrying out the crystal orientation has little deviation and shows a uniformly highest value. Specifically, the Alnico VIII alloy in which crystal coarsening is hard to obtain, can be well single-crystallised. Without cutting single-crystal magnet out of a part of a casting as before, the casting itself is of single crystal having a fixed orientation. The discovery that such heat treatment cycle can be easily embodied in in a continuous furnace is a feature of the present invention and very effective to the industry.

What is claimed is:

1. A method of producing a magnetically anisotropic single-crystal ferroalloy magnet having a composition consisting of 6-10% Al, 12-20% Ni, 20- 42% Co, .0-7% Cu, less than 6% Ti and the balance Fe, comprising subjecting a polycrystal oriented magnet of said composition to a series of more than two heat-treatment cycles, each said cycle comprising heating said magnet to 1250-1280 C. at a heating rate of 520 C. per minute and then slowly cooling to about 900 C., and then heat-treating the resultant single-crystal magnet in a magnetic field and aging said magnet.

2. A method of producing a magnetically anisotropic single-crystal ferroalloy magnet having a composition consisting of 6-10% Al, 12-20% Ni, 20-42% C0, .O7% Cu, less than 6% Ti and at least one taken from the group consisting of less than 1% Zr, less than 2% Si, and less than 3% Nb, comprising subjecting a polycrystal oriented magnet of said composition to a series of more than two heat-treatment cycles, each said cycle comprising heating said magnet to 12501280 C. at a heating rate of 5-20 C. per minute and then slowly cooling to about 900 C., and then heat-treating the resultant singlecrystal magnet in a magnetic field and aging said magnet.

References Cited UNITED STATES PATENTS 1,738,307 12/1929 McKeehan l48l.6 3,027,281 3/1962 Osborn et al. 1481.6 3,175,901 3/1965 Jesmont et a1. 148l03 3,219,495 11/1965 Steinort 1481.6 3,219,496 11/1965 Steingrover et al 1481.6 3,226,266 12/1965 Jesmont et a1. 148103 DAVID L. REC K, Primary Examiner.

N. F. MARKVA, Assistant Examiner. 

1. A METHOD OF PRODUCING A MAGNETICALLY ANISOTROPIC SINGLE-CRYSTAL FERROALLOY MAGNET HAVING A COMPOSITION CONSISTING OF 6-10% AL, 12-20% NI, 20-42% CO, .0-7% CU, LESS THAN 6% TI AND THE BALANCE FE, COMPRISING SUBJECTING A POLYCRYSTAL ORIENTED MAGNET OF SAID COMPOSITION TO A SERIES OF MORE THAN TWO HEAT-TREATMENT CYCLES, EACH SAID COMPRISING HEATING SAID MAGNET TO 1250-1280*C. AT A HEATING RATE OF 5-20*C. PER MINUTE AND THEN SLOWLY COOLING TO ABOUT 900*C., AND THEN HEAT-TREATIG THE RESULTANT SINGLE-CRYSTAL MAGNET IN A MAGNETIC FIELD AND AGING SAID MAGNET. 